The waste gas produced in blowing oxygen gas against molten iron in an oxygen-operated converter is generally a valuable gas that essentially contains carbon monoxide (CO) gas. The gas is recovered by cooling it and removing entrapped dust.
In the operation of a conventional converter waste gas disposing device, a suitable space is provided between a movable skirt, which is disposed between the opening of the converter and the hood, and the opening of the converter. In order to recover the CO gas (without burning it), it is necessary to prevent air from entering the converter through the gap between the skirt and the opening of the converter. This has been achieved by setting the gas pressure in the hood substantially equal to the atmospheric pressure. For this purpose, the gas pressure in the hood (hereinafter referred to as "the pressure in the converter" or "the converter pressure") is detected, and the flow rate of waste gas is controlled so that even if the production of gas in the converter varies irregularly, the pressure in the converter is maintained constant. In the case when the reaction in the converter is abruptly changed, for instance, by the addition of auxiliary raw materials during the smelting operation, the above-described converter pressure control sometimes fails to follow the change because of a delay in detecting the change, in transmitting the signal representing the gas pressure, or in responding at the control terminal. In this case, the waste gas may blow out of the converter through the gap between the skirt and the opening of the converter, or air may be forced into the converter through the gap to cause the combustion of the carbon monoxide gas, which is not economical. In order to eliminate this difficulty, heretofore the operator must manually adjust the gap by moving the skirt vertically.
The adjustment of the gap, however, sometimes changes the process parameters and makes the converter pressure control unstable or lowers the response characteristic. This phenomenon can be represented by a gas state equation and a gas pressure loss equation. That is, as for a process gain Kp the following relation can be established: ##EQU1## where Pa is the atmospheric pressure, Po is the converter pressure, and .gamma. is the converter's opening pressure loss coefficient.
The coefficient .gamma. is decreased as the gap between the skirt and the opening of the converter decreases. This is the reason why the above-described phenomenon occurs. This will be described in more detail.
A PI or PID controller for converter pressure control has control parameters such as a proportion gain (Kc) in a proportion operation, a time constant T.sub.I in an integration operation, and a time constant (T.sub.D) in a differentiation operation. These parameters are set to suitable values according to the process characteristic of a system to be controlled. Therefore, if when the process characteristic of the system to be controlled changes the parameters remain unchanged, then the control operation may become unstable.
In the above-described converter pressure control system, the variation of the gap between the skirt and the converter's opening changes the process characteristic. Considering only the proportion operation of the controller, even if a waste gas flow-rate control device (such as a secondary damper) is driven when the gas is sufficiently large, the pressure in the converter is changed only a little because air can readily go in and out of the converter through the large gap. Consequently, the proportion gain Kc of the controller may be large.
On the other hand, in the case where the gap is small, the pressure in the converter is greatly changed when the waste gas flow-rate control device is driven, and, therefore, the proportion gain Kc must be small. If the proportion gain Kc is large in the former case where the gap is large, the operation of the controller becomes slow, as a result of which the pressure in the converter is greatly changed. Accordingly, a large quantity of air goes into the converter through the gap so that the CO gas to be recovered is burned, or the waste gas blows out of the converter through the gap to cause air pollution. On the other hand, if the proportion gain Kc is small in the latter case where the gap is small, the pressure in the converter oscillates, and the control operation becomes unstable.
As is apparent from the above description, if the control parameters are maintained constant at all times, when the above-described difficulties occur when the gap between the skirt and the opening of the converter is changed. These difficulties may be eliminated by employment of a method in which the variation of the gap is detected by obtaining the quantity of air flowing into the converter, and the control parameters of the controller are adjusted according to the variation of the gap thus detected.
According to such a method, the control parameters can be adjusted for the gap between the skirt and opening of the converter. The difference pressure component .sqroot..vertline.Pa-Po.vertline. in the above-described relation (1) has not been a serious factor because, when the gap between the skirt and the opening of the converter is large, the variation of the converter is not more than 5 or 6 mm H.sub.2 O.
If the smelting operation is carried out with the skirt brought in close contact with the opening of the converter in order to increase the quantity of recovery of CO gas, the converter pressure is varied more than 100 mm H.sub.2 O with the same variation of the quantity of gas produced in the converter. Therefore, according to the above-described relation (1) the process gain becomes ten times as large. Accordingly, even if the control parameters are adjusted for the gap between the skirt and the opening of the converter, the pressure in the converter will oscillate. As a result, it becomes impossible to continue the smelting operation.